The clerk's office in the federal courthouse in Brooklyn was quarantined Tuesday after an envelope with powder in it was opened by an employee, but the substance was later determined to be "nonhazardous."

The envelope was from a pro-se litigant -- an individual pursuing a case on his or her own, without a lawyer -- and the correspondence inside contained the word "anthrax," a court official said. The FBI is investigating the envelope's source.

After the envelope was first opened, about 20 people were briefly quarantined inside. Three who were most directly exposed were decontaminated, but tests by the NYPD and the city Department of Environmental Protection later concluded the powder was not a danger.

Courthouse officials said they did not know if the envelope was addressed to a judge or the clerk's office. The U.S. District Court is located at 225 Cadman Plaza.